This is one of those questions that I have been struggling with and I am sure someone out there has experience of a similar situation who may be able to help point me in the right direction.

One of the sites I have responsibilities for is a small urban farm. Its on quite a small footprint with several covered wooden animal pens and toilets etc. There are a couple of shipping containers for storage etc. There are also a few large polytunnels which are used for classrooms, greenhouses and aquaponics.

As the site isn't that big our emergency evacuation plan has everyone leaving the site completely. This is also what our fire risk assessment recommends. There isn't really any part of the site which is suitable to use as a muster point. To allow a speedy evacuation the external gates have always been left unlocked all the time there are people on site.
However its been raised that as there are children and potentially vulnerable young people on site that we should be securing the perimeter from a safeguarding perspective. This is both to stop people getting onto the site, but also to help stop the children getting off the site. I agree to a large extent as I have never been completely happy with the lone working arrangements due to its urban location so both of these point to putting in better security.
The problem is that I can't seem to find anything that would work on the type of Palisade fencing that makes up the perimeter. We can't really use push bars as these wouldn't stop someone getting out. If it was internal I would just put an alarm on it, but that wouldn't work particularly well outside.

So that's all a very waffly way of seeing if anyone has come across a good, easy to use way of securing final exits where the final exit is external and on metal fencing?